The present invention relates to a method of forming a pattern by transferring an original mask onto a substrate such as a photomask to be exposed to light. More particularly, the present invention relates to a method of forming a pattern for preventing a defect produced in the original mask from being transferred to the substrate to be exposed to light. Furthermore, the present invention relates to a photomask for use in the method of forming a pattern.
In a lithography technique widely used in manufacturing processes of semiconductor devices and the like, it is a great matter of concern to reduce defects as well as to improve accuracy in size and arrangement of the pattern. In the light exposure technique for preparing and transferring the original mask, defects of the original mask are transferred throughout light exposure process as common defects. Therefore, conventionally, accurate inspection and repair have been made.
In a typical light-exposure technique called an optical reduction projection exposure method, a fifth- or fourth-fold mask is usually used. In the manufacturing steps of the photomask, a raster or vector scan exposure method using an electron beam or a laser beam is generally used. Furthermore, it is known that an photomask is manufactured by applying a technique in which an LSI pattern is reduced and transferred from an original mask using an optical reduction projection exposure method.
The defects in the original mask have been repaired by using various repair techniques to a level having no effect upon the transfer results. However, as a size of the LSI pattern comes closer to a resolution limit of the optical projection exposure method with miniaturization of the semiconductor devices, it has been difficult to repair the defects to the level having no effect upon the transfer results.
To repair the defects of the photomask, employed are a method of removing an opaque film by a laser beam, a method for forming an opaque film by depositing of a carbon film using a focused ion beam (FIB) and so forth. However, it is difficult to repair defects with an accuracy of 50 nm or less due to limitations in size and alignment of the beam. Furthermore, in the case where Ga ions etc. are implanted into a quartz substrate for repair, optical characteristics of the repaired portion and the peripheral region thereof are degraded. Alternatively, in the repaired portion, the opaque film is not formed with a desired sectional profile of the end portion. As a result, a transferred image is degraded.
In a conventional pattern formation method for forming an LSI pattern on the substrate to be exposed to light (hereinafter, simply referred to as "substrate") such as a photomask by use of the original mask, repairing the defects is a great matter of concern. It has become more difficult to repair the defects so as not to have an effect upon the transfer results with the miniaturization of the device. It is difficult to repair the defects with a sufficient repair accuracy after being transferred to the substrate because of limitations of a beam in size and alignment. In addition, some regions are degraded in optical characteristics after repair, or when an image is transferred onto the repaired portion, the transferred image is degraded. In particular, due to limitations in the defect repair technique, it is difficult to repair the defects produced in the pattern edge portion with a high accuracy.